Monarch Butterflies: Magnetic Field Guides Their Long Journey

Monarch butterflies, in the thousands, travel from North America to central Mexico each year in the fall and a recent study has shown that they use the Earth’s magnetic field to guide their way on their long journey. How monarch butterflies know where to go on this long-distance journey has been a wonder for many.

It has been known for a while that monarch butterflies use the sun to navigate their way south, but there has been a question about what they do when the sun is hidden by clouds. Cloudy days do not seem to thwart the monarch butterflies on their journey.

A team of scientists from the University of Massachusetts Medical School and the Worcester Polytechnic Institute carried out the study that has offered an explanation for how the butterflies know where to go in the absence of the sun. They showed that monarch butterflies use the Earth’s magnetic field for direction. The butterflies were shown to have light-sensitive magnetosensors in their antennae that can detect magnetic fields just like a compass.

The study report was published in Nature Communications. In the report, they described using a device that allowed them to alter the magnetic fields around a box that contained the monarch butterflies. These were called simulator studies in that the experimental set up was similar to a flight simulator that human pilots use to learn how to fly an airplane. In the flight simulator box, the butterflies turned in the direction that would be considered “south” as the direction of the experimental magnetic fields were moved and switched. The monarchs turned south toward the equator initially, as they should, but when the magnetic field was reversed, for example, they turned north instead.

It has been suspected for a while that monarch butterflies use the magnetic field to guide their long journey south each year, but what made this particular experiment work to provide the evidence was the use of ultraviolet light (light that was 380 nm to 420 nm in wavelength) in the experimental set up. The magnetosensors in the antennae contain cryptochromes, which are molecules that respond to light in the ultra-violet range. As a control, the researchers covered up the antennae of some of the butterflies with black paint so that they could not respond to light. The butterflies with black painted antennae showed a low degree of directionality, which indicated the magnetosensors responding to the ultraviolet light was actually part of the mechanism for navigation using the magnetic field.

Monarch butterflies, as animals that use the magnetic field for navigation, join many other species that also have been shown to use the magnetic field for determining directions. Other insects, such as honeybees, turtles, birds and even amphibians and reptiles have been shown to use the magnetic field for navigation. Whereas the navigator device for the larger animals is in the brain, monarch butterflies were shown to have their “compass” in their antennae. A concern for all of these animals has been raised because humans have been “polluting the airwaves” with communication electromagnetic noise from cell phones and other devices.

While the monarch butterflies may use the sun primarily for navigation, this ultraviolet light sensory system that allows detection of the magnetic field is used to augment the primary system based on detecting the sun. Learning that the long journey of monarch butterflies is guided by the magnetic field of the Earth only adds to the wonder of these beautiful insects.